The present invention relates to an improved method for injection-molding parts of plastic materials. In particular, the present invention relates to the production of parts having large dimensions and heavy weight such as the wash tubs of household clothes washing machines, for example the type disclosed in EP 0 601 347 and U.S. Pat. No. 5,329,791.
When production volumes of molded parts are increased, increasing the number of injection molding presses for coping with such volumes would require considerable amounts of money to be invested. To avoid such an increase it would be desirable for the dwelling time of the molded parts in the respective mold to be made as short as possible. The fulfillment of such a requirement is opposed however, in the case of particularly large and heavy parts, by the speed at which the plastic material cools down after injection. The need in fact arises for the ejectors provided in the mold, which exert a localized pressure on the surface of the molded part in view of knocking it out of the mold, to be prevented from deforming it or breaking through it when the plastic material is not sufficiently stiff yet.